1. Field of the Invention
The present invention relates to a radiation image detector in which radiation detection devices for detecting radiation are at least linearly arranged.
2. Description of the Related Art
In recent years, FPD's (flat panel detectors), which can directly convert X-ray information into digital data, have become adopted for practical use (please refer to U.S. Pat. No. 5,744,807, for example). In the FPD, an X-ray sensitive layer is arranged on a TFT (thin film transistor) active matrix substrate. The FPD's rapidly became popular, because they have advantages over conventional imaging plates. Specifically, in the FPD's, it is possible to immediately check images and to check not only still images but motion images (video images, moving images or the like).
FIG. 8 is a schematic diagram illustrating the configuration of a conventional FPD. As illustrated in FIG. 8, the conventional FPD includes a multiplicity of radiation detection devices 202 that are two-dimensionally arranged, each including a TFT switch 201. Further, each of the radiation detection devices 202 is connected a scan line (scan wire) 203 for transmitting a control signal for controlling ON/OFF of the TFT switch 201 and a data line (data wire) 204. Signals detected by the radiation detection devices 202 are output to the data lines 204 through the TFT switches 201. Further, a gate driver 205 is connected to the scan lines 203 and a readout circuit 206 is connected to the data lines 204. The gate driver 205 outputs control signals for controlling ON/OFF of the TFT switches 201.
The readout circuit 206 includes charge amplifiers 207 and a multiplexer 208. The charge amplifiers 207 detect charge signals (electric charge signals) that have flowed into the data lines 204 as voltage signals. The multiplexer 208 sequentially switches columns of radiation detection devices and sequentially outputs signals detected by the radiation detection devices line by line.
The readout circuit 206 performs so-called correlated double sampling (CDS). FIG. 9 is a circuit diagram illustrating the readout circuit 206 in detail. As described above, the readout circuit 206 includes the charge amplifiers 207 and the multiplexer 208. Further, the readout circuit 206 includes a first sampling circuit 209, a second sampling circuit 210 and a differential amplifier 211. The first sampling circuit 209 samples the voltage signals output from the charge amplifier 207 and obtains a kTC noise component of the charge amplifier 207 (hereinafter, referred to as kTC noise component). The second sampling circuit 210 samples the voltage signals output from the charge amplifier 207 and obtains a data component. The differential amplifier 211 outputs a difference between the signal of the kTC noise component, which has been obtained by sampling by the first sampling circuit 209, and the signal of the data component, which has been obtained by sampling by the second sampling circuit 210.
Next, the operation of the conventional FPD will be described.
First, when a radiation image is recorded, the FPD is irradiated with radiation. Then, charges corresponding to the dose of the radiation are generated in a radiation sensitive layer of the radiation detection device. A radiation image is recorded by accumulating the charges generated in the radiation detection device.
Next, the action of reading out, by the readout circuit 206, the radiation image recorded in the FPD will be described with reference to FIGS. 8 and 9 and a timing chart illustrated in FIG. 10. In FIG. 10, gate signal Gate, control signal CA_Reset, control signal SH1, control signal SH2, output signal CA_out, ground potential GND, output signal SH1_out and output signal SH2_out are illustrated. The gate signal Gate is output from the gate driver 205 to the scan line 203. The control signal CA_Reset controls a reset switch of the charge amplifier 207. The control signal SH1 controls a switch of the first sampling circuit 209. The control signal SH2 controls a switch of the second sampling circuit 210. The output signal CA_out is output from the charge amplifier 207. The ground potential GND is the ground potential of the charge amplifier 207. The output signal SH1_out is output from the first sampling circuit 209. The output signal SH2_out is output from the second sampling circuit 210.
First, the reset switch CA_Reset of the charge amplifier 207 is opened and the charge amplifier 207 operates in storage mode (accumulation mode). Next, a control signal is output from the gate driver 205 and TFT switches 201 of the radiation detection devices 202 in line L1, which is the rightmost line in FIG. 8, are turned on. Then, charges stored in the radiation detection devices 202 are output to the charge amplifiers 207.
Then, the charge amplifiers 207 start accumulation of the charges. Then, immediately after the charge amplifiers 207 have started accumulation of the charges, the switch SH1 of the first sampling circuit 209 and the switch SH2 of the second sampling circuit 210 are turned on. Then, the switch SH1 of the first sampling circuit 209 is immediately turned off and the signal of the kTC noise component is obtained by sampling by a capacitor of the first sampling circuit 209.
After then, the charge amplifiers 207 accumulate charges only for a predetermined time period. Then, immediately before the reset switch CA_Reset of the charge amplifier 207 is short-circuited, the switch SH2 of the second sampling circuit 210 is turned off. Then, the signal of the data component is obtained by sampling by the capacitor of the second sampling circuit 210.
Next, a switch device 208a in the multiplexer 208 is turned on. Then, the multiplexer 208 outputs the signal of the kTC noise component, which was obtained by sampling by the first sampling circuit 209, and the signal of the data component, which was obtained by sampling by the second sampling circuit 210.
Then, the signal of the kTC noise component output from the first sampling circuit 209 and the signal of the data component output from the second sampling circuit 210 are input to the differential amplifier 211. In the differential amplifier 211, the signal of the kTC noise component is subtracted from the signal of the data component. Accordingly, an image signal on which correlated double sampling has been performed is obtained.
Then, switch devices 208b through 208d in the multiplexer 208 are sequentially turned on. The signal of the kTC noise component obtained by sampling by the first sampling circuit 209 and the signal of the data component obtained by sampling by the second sampling circuit 210 are sequentially output from the multiplexer 208 in a manner similar to the aforementioned process. The differential amplifier 211 sequentially obtains differences. Accordingly, image signals are sequentially obtained. As described above, signals detected by the radiation detection devices 202 in line L1 of the FPD are read out.
Further, in a manner similar to the aforementioned process, signals detected by the radiation detection devices 202 in lines L1 through L4 of the FPD are read out line by line.
Here, when the image signals are read out line by line as described above, an output from the radiation detection device on which readout is performed last among the radiation detection devices in each line connects to GND through surrounding circuits and leakage current flows. Consequently, the GND potential fluctuates. Or, the output from the radiation detection device connects to GND by parasitic capacity and the GND potential fluctuates.
Further, the fluctuation of the potential depends on an output voltage from the radiation detection device on which readout is performed last among the radiation detection devices in each line. Especially, when the radiation detection device is arranged in an area, such as an empty portion without any subject, which is irradiated with a high dose of radiation, the output voltage becomes high. Therefore, an influence from the GND fluctuation becomes significant.
More specifically, as illustrated in the timing chart of FIG. 10, if the output from the radiation detection device on which readout is performed last among the radiation detection devices in a predetermined line ((N-1)th row) is an output voltage corresponding to a high dose, GND at the time of reading out the following line (N-th row) fluctuates. Further, the magnitude (size) of the signal on which correlated double sampling has been performed becomes smaller by the offset fluctuation amount.
The fluctuation of the GND potential influences all of the charge amplifiers 207 in the readout circuit 206. Therefore, image signals for the entire one line are influenced by the fluctuation.
To prevent such influence of the GND fluctuation, the timing of correlated double sampling may be shifted (delayed) until the fluctuated GND potential becomes stable. However, if the timing is shifted in such a manner, the readout time becomes long and it becomes impossible to increase a frame rate.